The present disclosure relates to a monitoring method and a corresponding monitoring device for an electrostatic coating installation, for example, for coating components with a coating agent which is electrically charged by means of a high voltage device.
The relevant industrial standards (e.g. EN 50176, EN 50348) stipulate that such electrostatic coating installations must comprise monitoring devices, to prevent an impermissible rise in electrical operating current. These monitoring devices continuously check the actual value of an electrical operating variable (e.g. operating current) of the high voltage device against a maximum and/or minimum permissible limit value.
In voltage-controlled operation of the electrostatic coating installation, the operating current is generally compared with a maximum permissible limit value.
If the possible alternative of a current-controlled operation is used for the electrostatic coating installation, on the other hand, the operating voltage is as a rule compared with a minimum permissible limit value for the voltage.
If the measured electrical operating variable exceeds or falls below the specified limit value, depending on the mode of operation (current-controlled or voltage-controlled), the high voltage supply is immediately switched off as a safety measure.
The above-described conventional monitoring method for an electrostatic coating installation operates satisfactorily when processing high resistance varnish paints (>400 kΩ on the Ransburg scale). When coating motor vehicle body components, however, low resistance varnish paints (<300 kΩ on the Ransburg scale) are increasingly used, which often have a high solids content, which holds a certain safety risk since the limit values for the safety thresholds have to be set increasingly higher because the ratio between the coating current flowing via the component to be coated on the one hand and the installation current flowing away via the rest of the coating installation on the other hand is always less favourable due to the low resistance of the varnish paint used.
This problem is further aggravated by fluctuations in the electrical resistance of the varnish paint between different batches or different production series. For instance, the ratio between coating current and installation current was hitherto 10:1, whereas with low resistance varnish paints the ratio of coating current to installation current may be 1:2. Installations even exist in which the coating current amounts to 2 μA and the installation current to 50 μA. Thus, varnish paints with a high solids content exhibit low resistance, for example. Furthermore, solvents and additives make the varnish paint more or less conductive. In addition, special effect particles (e.g. metallic flakes) form a conductive path when the varnish paint is circulating. This results in the fluctuations of the installation current having a significantly more severe effect.
In addition, both the coating current and the installation current are dependent on the coating voltage. This makes itself felt disadvantageously in particular in the case of a large installation current, since if for example coating is carried out at a number of different voltages, the limit value for monitoring has to be set at the highest voltage used. This results in the noise ratio between the normal operating current and the limit value for safety switch-off becoming ever higher, the lower the coating voltages.
A monitoring method is known from DE 199 03 824 A1 in which the limit value is not however adapted as a function of electrical operating variables.
In addition a current limiting circuit is known from DD 31812 A in which the limit value has however to be individually set for each type of coating, the shape of each part to be coated and each degree of wetting. Automatic adaptation of the limit value is thus not known from this document.
This also applies to US 2008/0203198 A1, in which the limit value has likewise to be set as a function of the component to be coated.
Nor is it known from US 2007/0227445 A1 to set the limit value automatically, as a function of electrical operating variables of the coating installation.
Finally, U.S. Pat. No. 4,073,002 does not disclose any limit value adaptation at all. Rather, this document merely provides a current-limiting resistance, which automatically leads to stabilisation.
The object of the present disclosure is therefore to provide an improved monitoring method and an improved monitoring device for an electrostatic coating installation.
It is in particular desirable to avoid the above-described problems of the prior art.